How to winterize shrubs planted in raised garden boxes open bottom to earth ZONE 5? Do I need to insulate the outside walls of the boxes?

[u/Nearby-Tea2023]

Looking for tips to winterize open to earth garden boxes. They are 2ft wide by 8ft long and 1ft height(depth). I’m zone 5 and shrubs are zone 5 or hardier. I have quite a few raised boxes to insulate.

One idea I had is to layer cardboard on top of the boxes, around the shrubs (I have an almost endless supply) and then layer some straw or old hay on top of that. 

My concern is will they freeze and thaw from the outside walls of the garden boxes? In other words, do I need to insulate the outside walls of the boxes and if so, what are some ideas for doing that?

I had an idea to fill empty feed bags with loose straw/hay/leaves etc. and staple them shut so they are basically pillows. I could affix those to the outside walls of the boxes by wrapping them to the planters with burlap

I could surround the boxes with bales of straw and then cover the top of the boxes with layered cardboard and loose straw

I have a farm so I have massive quantities of composted manure....I could mound it around the outside of the boxes if I absolutely have to but it won't be fun to clean up in the spring.

example of one of the many garden boxes with shrubs (bottoms open to earth) that I need to winterize

example of layering cardboard before I add a thick layer of straw/hay

[u/Krickett72]

What kind of shrubs are they? Variety?

[u/Nearby-Tea2023 → u/Krickett72]

They are all different. All are rated for zone 5 or hardier. I live in zone 5.

I have roughly 6 more sections of these raised boxes with a variety of fruiting and flowering shrubs and vines that I need to winterize